1. Field of the Invention
The present invention relates to a programmable logic integrated circuit which can interface with any of, for example, TLL level and C-MOS devices.
2. Brief Description of the Prior Art
A programmable logic integrated circuit consists of generally an input buffer segment, a programmable logic segment and an output buffer segment. A specific example of this type of programmable logic integrated circuit is illustrated in FIG. 3. As illustrated, an input buffer segment 1 is comprised of input buffers 10.sub.-1, 10.sub.-2, . . . , 10.sub.-n, and an output buffer segment 2 comprises output buffers 20.sub.-1, 20.sub.-2, . . . , 20.sub.-n. A programmable logic segment 3 comprises a matrix unit 31 formed from a plurality of lines and rows, OR circuits 32.sub.-1 ; 32.sub.-2, . . . , 32.sub.-n, and exclusive OR circuits 33.sub.-1, 33.sub.-2, . . . , 33.sub.-n. A transistor (not shown) is positioned at each junction between the line and row of the matrix 31, and according to the program, the transistors at necessary junctions are kept active, while those at the remaining junctions are blocked. In this manner, AND circuits are formed using the rows with active transistors as inputs and lines with active transistors as outputs. Each of these AND circuits is indicated at A. The illustrated programmable logic integrated circuit has been implemented with bipolar transistors and recently has been implemented using C-MOS transistors for reduced consumption current.
As mentioned above, the early programmable logic integrated circuit was implemented with bipolar transistors and the peripheral integrated circuit was also usually bipolar. Therefore, the interface of the input and output buffers was naturally interfaced at the TTL level.
However, to reduce the consumption of current, programmable logic ICs of C-MOS construction have been developed. In consideration of the bipolar integrated circuit to be interfaced, the input and output buffers are designed to interface at the TTL level. If one attempts to use C-MOS integrated circuits for the peripheral integrated circuit, too, entire electronic circuit board consists of of C-MOS integrated circuits and the peripheral C-MOS IC must be connected to the conventional programmable logic integrated circuit. The device itself is of C-MOS construction but the interface is of TTL construction, and the problem of a conflict of level occurred. Therefore, a level-changing circuit must be provided in order to convert a system comprising a C-MOS device and a TTL peripheral integrated circuit to an all C-MOS system, it is necessary to redesign the board requiring additional development time and cost.
The present invention is intended to solve this problem by providing a programmable logic IC which can interface at either TTL and C-MOS levels.